The present invention relates generally to measurement devices and, more particularly, to devices which measure anodic capacity in metallic layers.
The application of conductive coatings to rebar reinforced concrete is a technique used to arrest the degradation of reinforced concrete structures. By this technique, a conductive coating is applied to a concrete structure to form a galvanic sacrificial coating. The coating serves as an anode in both passive and active impressed (rectified AC) or galvanic (DC) cathodic protection. Typical conductive coatings in use today include zinc and zinc/aluminum alloys. These coatings are especially suited for use in areas where an electrolyte can leach into the concrete, for example in underground and underwater structures. The coatings are typically applied to the structure using a thermal spray system in which the coating is melted under intense heat and propelled onto the target structure to solidify.
The service life or "anodic capacity" of the coating can be roughly approximated by the thickness of the coating. Accordingly, during the coating application, measurements of applied coating thickness can be useful in estimating whether adequate anodic capacity is being applied. Similarly, during the service life of a coated structure, the remaining anodic protection can be estimated by measuring the remaining coating thickness at regular intervals.
Because so many reinforced concrete structures benefit from this form of protection there is an increasing need to accurately measure the remaining anodic protection capacity of such coatings at various points during the life cycle of the anodic layer. Conventionally, however, the measurement of applied anodic capacity and remaining anodic capacity typically involved measuring anode thickness using destructive techniques in which the protective coatings are damaged and subsequently repaired. Several such destructive techniques are described below.
One way in which the initial thickness of an anodic layer has conventionally been measured is to place masking tape over the uncoated concrete, apply zinc to the concrete, and then remove the zinc-covered tape. The thickness of the zinc layer can then be measured by removing the tape from the concrete, using calipers or a micrometer to measure the thickness of the zinc and tape, and subtracting the thickness of the tape. In the same way, a steel coupon can be adhered to the concrete prior to spraying the zinc and subsequently removed. The zinc and steel thickness are then measured using an electronic or magnetic gauge, or using the mechanical gauges described earlier for measuring the zinc on the masking tape.
Another way in which the thickness of a conductive coating can be determined is by measuring electrical resistance. Using a four conductor probe configured to measure surface resistance one can accurately measure the thickness of a conductive coating applied to a nonconductive substrate. The four contact surface probe can be used to apply a known current to the surface of the coating and resistivity can be determined by measuring the voltage drop across a known distance. The thickness of the coating can then be correlated to resistivity. However, this technique requires that electrical contact be made with the surface of the coating, which may prove difficult when measuring, for example, the remaining anodic capacity of partially corroded coatings that are covered with non-conductive corrosion. Thus, to measure the remaining anodic capacity of an anode that has been in service would require that the coating be removed for visual inspection to determine how much of the pure zinc layer remains.
These destructive approaches are of course undesirable since they reduce the effectiveness of the protective zinc layer by creating openings. In order to overcome this problem, non-destructive methods for determining zinc layer characteristics have been proposed. For example, eddy currents have been used to measure conductive material thickness per se in a nondestructive manner.
This method utilizes a first coil to generate a magnetic field which induces an eddy current in the conductive material. The eddy current, in turn, creates a second magnetic field which is sensed by a second coil. The voltage induced in the second coil can then be correlated to the thickness of the conductive material. Simply knowing the thickness of the material, however, is insufficient to accurately determine its remaining anodic capacity since other factors, such as the density of the material, also impact on this determination.
Thus, conventional methods and apparatuses fail to provide a nondestructive way of determining how much longer the protection provided by the zinc layer to the concrete will last. Moreover, known instruments do not measure the anodic capacity of an anode and represent this quantity in standardized units that are meaningful to an operator.
Furthermore, conventional eddy current devices cannot be calibrated to measure the anodic capacity of an anode, for example, using the coating characteristics of the applied conductive material.